Light Detection and Ranging, or “Lidar” (also referred to as Laser Detection and Ranging (LADAR)) has typically involved propagating a pulse of laser light to an object and measuring the time it takes for the pulse to scatter and return from the object. Typically, a Lidar system comprises a laser that fires pulses of laser light at a surface. A sensor on the Lidar system measures the amount of time it takes for each pulse to bounce back. Since, light moves at a constant and known speed (˜3×108 meters per second in air), the Lidar system can calculate the distance between itself and the target.
Conventional Lidar systems accumulate statistics with multiple laser pulses to determine the range to a target. These systems typically employ a constant pulse or code repetition rate. And, the pulse or code repetition time can generally not be less than the round-trip time of flight of the laser pulse to the target. Otherwise, the conventional Lidar system produces range ambiguity.
So, in many instances, conventional Lidar systems modulate laser pulses prior to transmitting it into a region of interrogation. The signal is scattered off hard targets and/or volumetrically distributed scatterers, such as molecules or other particles. And, the time of flight delay is measured by detecting the delay in the modulation of the received signal.
Generally, the pulse modulations are necessary but often result in inefficient use of power and expensive laser architectures, such as those employing Q-switched lasers. Additionally, if signals are received from multiple ranges (e.g., if an extended volume of distributed scatterers are sensed), signals scattered from near ranges may be much stronger than signals scattered from far ranges. This places a burden on the dynamic range of the conventional Lidar system and can limit detection performance. For example, if the Lidar system is tuned to detect far range targets, saturation from near range return signals and temporal tails in the detector response may overwhelm the far range signals. And, if the system is tuned to observe near range targets, far range signals may be beneath the noise floor making detection of farther targets difficult if not impossible.
Another disadvantage of conventional Lidar systems is the processing that may be required for detection, especially within a cluttered environment. The waveform of the detected signal needs to be processed to determine a delay relative to the transmitted waveform. When signals from multiple ranges are to be detected within the received waveform, the conventional Lidar system needs to isolate multiple delayed copies of the signal modulation so as to distinguish between the targets, which results in additional processing.
And, in applications where small objects are detected within a large solid angle search area, laser pulses need to completely fill the volume, resulting in high pulse repetition frequencies (PRF's), high laser power, or lower scan rates. If scan rates result in higher repetition rates, the time between pulses may be higher than the round trip time-of-flight for laser pulses to the more distant targets. In other words, multiple laser pulses will be in the air at any given point in time. And, more complex encodings on the laser signal may be needed so that the detector can determine which laser pulse it is receiving.